1. Field of the Invention
The present invention is in the field of manufacturing a resistor, particularly an arc suppression resistor employed in a cathode ray tube to suppress the harmful influence of an arc discharge which may accidentally occur within the cathode ray tube.
2. Description of the Prior Art
In the prior art, a cathode ray tube for use with a television receiver is designed and manufactured with great care in order to avoid a discharge occurring within the cathode ray tube, and particularly, to prevent an arc discharge from occurring between the electrodes of an electron gun or between an electrode and some other portion. However, it has been found that a discharge occurring due to various accidental causes can not be avoided completely. If the cathode ray tube is not provided with proper means for avoiding such a defect, an extremely large current flows along the discharge path, burning out the electrodes, breaking the interconnection between the electrodes due to the burning of the connection wire, or damaging the circuitry or the like of the television receiver, and the like. To cope with this problem caused by the discharge current, there has been suggested a method which is known as the soft-flash method. In this method, there is provided an inner conductive film of high resistance on the inner surface of the tube envelope. The discharge energy is thus dissipated within the conductive film. Alternatively, it has been proposed to employ a high value resistor as the conductive connection wire for connecting the electrodes that form the electron gun.
FIG. 1 illustrates an example of a cathode ray tube that employs a resistor having a high resistance according to the latter method. As illustrated in FIG. 1, the cathode ray tube has an electron gun 1 located within a neck portion 3 of a tube envelope 2. The electron gun 1 comprises a cathode K and first to fifth grids G1 to G5 in that order. The third to fifth grids G3 to G5 constitute a unipotential type main electron lens. The third and fifth grids G3 and G5 have applied a high voltage, that is, an anode voltage similar to that applied to the phosphor screen (not shown). The third and fifth grids G3 and G5 are energized as follows. The free end of a flexible metal lead member 6 is placed in resilient contact with an inner conductive layer 5. The inner conductive layer 5 is made of a graphite coated layer or the like coated on the inner surface of a funnel portion 4 of the tube envelope 2 and which has applied to it a high voltage. The flexible metal lead member 6 is attached to the fifth grid G5. Further, the fifth and third grids G5 and G3 are connected to each other by a resistor having a high resistance, that is, an arc suppression resistor R, thus energizing the third and fifth grids G3 and G5. Other electrodes such as the cathode K and the first, second and fourth grids G1, G2 and G4 are respectively connected to corresponding terminal pins 8 through conductors. The terminal pins 8 are extended through a stem 7 which is sealed to the end portion of the neck portion 3. Thus, the cathode K and the first, second and fourth grids G1, G2 and G4 are energized through the various terminal pins 8. In this case, particularly the focusing electrode is applied with a low voltage, that is, the fourth grid G4 and the corresponding terminal pin 8 are similarly coupled through an arc suppression resistor R. In the normal state, no current flows through these arc suppression resistors R so that the characteristics of the cathode ray tube are not affected. When a current produced by an arc discharge occurs, these arc suppression resistors R can produce a current suppression effect.
Arc suppression resistors R may be formed by mixing and sintering alumina, clay and graphite powder as disclosed, for example, in Japanese Patent Application No. 61-43205. This previously proposed arc suppression resistor will be described briefly hereinafter.
The known arc suppression resistor is manufactured as follows. A columnar shaped molded product is made from a ceramic material such as alumina containing carbon and is baked in an oxygen atmosphere. Then, only the carbon from the surface thereof is removed as carbon dioxide to thereby enable the baked ceramic product to have a high resistance due to the presence of a ceramic insulating layer made of alumina on the surface thereof. Since the carbon remains on the inside of the above baked ceramic product, the inside of the baked ceramic material has a ceramic resistor core made of alumina and carbon having a predetermined resistivity. In the above described arc suppression resistor, the graphite powder functions as a conductive element. Since a high resistance resistor can achieve a substantial arc suppression effect and the resistance value thereof can be controlled with ease, the arc discharge current can also be controlled very readily.
The aforementioned arc suppression resistor, however, employs graphite that essentially releases a large amount of gas so that when the resistor is heated by electrical current from the arc discharge, it releases gas. In the worst cases, it gradually releases gas even when in the static state. Thus, the conventional arc suppression resistor hinders the proper functioning of the electron emission cathode.
The gas is released because the ceramic insulating layer covering the surface of the above described arc suppression resistor is inherently porous. In other words, upon manufacturing, when the graphite near the surface of the alumina ceramic molded product containing graphite is baked to form the ceramic insulating layer, a large number of pores are formed through the ceramic insulating layer to release the burning gases therethrough.